DESCRIPTION: (Scanned from the applicant's abstract) Recently, this laboratory biochemically purified and molecularly isolated a unique, cytosolic form of mammalian adenylyl cyclase, the soluble Adenylyl Cyclase (sAC). sAC and the more widely studied, G protein responsive, transmembrane adenylyl cyclases (tmACs) define distinct cAMP signaling pathways within eukaryotic cells. While tmACs are modulated by heterotrimeric G proteins in response to extracellular signals which act through seven-transmembrane spanning receptors, sAC is directly regulated by bicarbonate ions suggesting it functions as the physiological CO2 chemosensor. sAC is most abundantly expressed in male germ cells, and likely mediates a number of the bicarbonate-induced, cAMP-mediated changes sperm must undergo prior to fertilization; such as capacitation, hyperactivated motility, and the acrosome reaction. During maturation from pachytene spermatocytes to spermatozoa, full-length sAC protein (187 kD) seems to be proteolytically processed via 150 kD intermediates to three distinct mature isoforms of 120 kD, 48 kD and 45 kD. The molecular details of these sAC isoforms are unknown. Individual sAC isoforms seem to be targeted to distinct subcellular compartments suggesting they generate second messenger directly and independently at the various subcellular sites of cAMP action. Additionally, proteolysis represents a mechanism for activating sAC. The specific activity of full-length sAC protein is at least 20 times lower than the enzymatic activity of a C-terminal truncation consisting of the amino terminal 53 kD. These data suggest the presence of an autoinhibitory domain C-terminal to the catalytic region. With the help of this grant, we plan to molecularly identify and structurally and biochemically characterize the physiological sAC isoforms in male germ cells. These studies will ultimately lead to an understanding of their specific biological roles. A long-term objective of this work is development of highly specific pharmacological reagents targeting individual isoforms as a novel method of contraception or to treat infertility.